1. Field of the Invention
The invention relates to a torque sensor for measuring a torque applied to a rotating shaft and particularly to a downsizing technique of a torque sensor used for various rotating shaft-equipped apparatuses, such as automobiles and industrial equipment, which can be improved in performance by feedback control of a torque.
2. Description of the Related Art
A system has been used in which the torque of a rotating shaft provided in automobiles and industrial equipment is measured and feedback control is carried out based on the measured value. In particular, engine control units and electromotive power steering devices are typical examples of applications of the system. Also in electric operation-assisted bicycles, a torque assist is carried out by a motor according to the force on the pedal of a person who rides on the bicycle, and, in this case, a sensor for measuring the torque is indispensable. Further, a device for measuring a cutting torque applied to a workpiece for control has been put to practical use for use in machine tools such as NCs. Thus, torque sensors for measuring the torque of the rotating shaft are of growing importance for high-performance control.
In general, the application of force in the direction of compression to a material having magnetostrictive properties causes a change in magnetization of the material having magnetostrictive properties, and the relative permeability is reduced. A magnetostrictive torque sensor using a rotating shaft having magnetostrictive properties has hitherto been well known as a technique for measuring the torque utilizing this principle.
Methods usable for this are described in Japanese Patent Laid-Open Nos. 83769/1995, 221940/1994, and 273247/1994. Among them, the first method is described in Japanese Patent Laid-Open No. 83769/1995. Specifically, the first method is a technique in which a slit, a groove or the like is provided in a direction at an angle of ±45 degrees to the rotating shaft to detect a magnetic variation attributable to magnetostrictive properties. Upon the application of a torque to the rotating shaft, force of compression occurs in one 45-degree direction of the rotating shaft, and tensile force occurs in the other 45-degree direction of the rotating shaft. Therefore, when a slit is provided in a direction to which compression force is applied, the compression force in this direction is concentrated, resulting in reduced relative permeability. The magnitude of the torque is detected by detecting this state of change as a change in inductance by utilizing an externally wound coil. This method is characterized in that the torque can be stably detected independently of the angle of rotation of the rotating shaft.
The second method is a technique disclosed in Japanese Patent Laid-Open No. 221940/1994 in which a change in the magnetized state in a direction at an angle of ±45 degrees to the rotating shaft having magnetostrictive properties is detected with an 8-shaped coil. The third method is a technique disclosed in Japanese Patent Laid-Open No. 273247/1994 in which two sets of coils prepared by folding back waved wiring into a ladder form are disposed respectively in ±45-degree directions relative to the rotating shaft to detect a change in the magnetized state in the direction depending upon the torque. The second and third methods are characterized in that there is no need to work the rotating shaft so far as the rotating shaft has magnetostrictive properties.
The above methods, however, suffer from the following problems.
The first method requires the provision of a groove in the rotating shaft or the application of a thin film having magnetostrictive properties, making it necessary to work the shaft per se. Therefore, when the rotating shaft cannot be worked without difficulties as in the case of engine torque or transmission torque, the application of the first method is disadvantageously difficult.
The second and third methods are characterized in that there is no need to work the shaft. In these methods, however, a change in the angle of the rotating shaft disadvantageously causes a change in the region of the rotating shaft to be measured, disadvantageously posing a problem that the magnetic properties vary depending upon the angle of rotation. Therefore, even when the torque applied is constant, the output of the torque sometimes varies depending upon the angle of rotation.